This assay is performed using the HemosIL von Willebrand Factor Antigen Kit on the Beckman Coulter ACL TOP. This is a latex immunoassay method using microlatex particles coated with specific rabbit polyclonal antibody directed against von Willebrand factor (VWF). In the presence of VWF antigen, antibody-coated latex particles agglutinate to form aggregates of diameters greater than the wavelength of the light passing through the sample and more light is absorbed as aggregation increases. The increase in absorption is proportional to the concentration of VWF antigen present in the sample.(Veyradier A, Fressinaud E, Sigaud M, et al: A new automated method for von Willebrand factor antigen measurement using latex particles. Thromb Haemost 1999;81:320-321)